(1) Field of the Invention
This invention relates to an opaque silica glass article and a process for producing the same. More particularly, it relates to an opaque silica glass article comprising a transparent portion and an opaque portion, and having good heat insulating property and good surface smoothness, and to a process for producing the opaque silica glass article by melt-forming together a raw material for the opaque portion and a raw material for the transparent portion into an article of an arbitrary shape.
(2) Description of the Related Art
An opaque silica glass article has good heat-insulating property, i.e., is capable of cutting-off heat rays transferring as radiant heat. In the case where the silica glass article contains a salient amount of fine bubbles uniformly distributed therein, its heat-insulating performance is superior.
One example of the opaque silica glass article is a flange provided at the base of a furnace tube used as a furnace for heating a silicon wafer, as illustrated in FIG. 1. A heating furnace illustrated in FIG. 1 has heretofore used widely for heating a silicon wafer, which comprises a heating element 1, a furnace tube 2, a boat 4 for supporting silicon wafers 3, an insulating cylinder 5 and a base 6. A flange 9 is provided at the base of the furnace tube 2. The flange 9 is made of opaque silica glass and welded together with the furnace tube 2 by an oxyhydrogen flame. The flange 9 has a function of heat insulation for cutting off heat transferring to the base 6 and a packing 7, which have a poor heat resistance. A desired atmosphere can be kept within the furnace tube 2 by the seal by means of packing 7 between the flange 9 and the base 6. Opaque silica glass is widely used in many fields including the flange of a heating furnace.
The opaque silica glass article is usually made by a method for heating a powdery siliceous raw material to melt and vitrify the raw material. The method for heating the raw material includes, for example, Verneuil's method wherein the raw material is subjected to flame fusion by using an argon-oxygen plasma flame or an oxyhydrogen flame, and a vacuum melting method wherein a vessel is charged with the raw material and the raw material is heated and melted in vacuo.
As the raw material for the opaque silica glass article, natural silica rock or stone, and rock crystal of a low quality level have heretofore been widely used. These raw materials contain a multiplicity of fine bubbles therein, and, when the raw materials are melted for vitrification, the bubbles remain within the glass to yield opaque silica glass articles.
In recent years, LSI is being highly integrated in the field of a semiconductor, and thus a raw material with a high purity of an opaque silica glass article is eagerly desired. A most typical example of the silica glass article is the above-illustrated flange of a furnace tube used in a furnace for heating a silicon wafer. However, natural raw materials used for the production of an opaque silica glass article contain a salient amount of impurities as well as a salient amount of fine bubbles, and the bubbles are very difficult to remove. Namely it is difficult to obtain a raw material with a high purity by purification. On the other hands, a rock crystal with a relatively high purity contains a minor amount of fine bubbles therein in the crystal, and therefore, even when the rock crystal is melted, the degree of opaqueness is not enhanced and the resulting silica glass article is translucent.
To solve the above-mentioned problems of the prior art, many proposals have been made. For example, a process has been proposed wherein an amorphous silica with a high purity which contains reduced amounts of an alkali metal, an alkaline earth metal, iron and aluminum, and a salient amount of fine bubbles, and has a silanol group as a vaporizable ingredient contained uniformly at a specific concentration is subjected to flame fusion (Japanese Unexamined Patent Publication (abbreviated to "JP-A") H6-24711). However, only silica glass articles having a simple shape such as an IC (Integrated circuit)-sealing silica filler and a matrix ingot for silica glass powder can be directly produced, and after-treatments such as after-shaping by lathing are necessary for the production of silica glass articles with a complicated shape such as a flange-form, a ring-shape, column, square pillar or hollow-square pillar. Utilization of the raw material is low in the production of silica glass articles with a complicated shape, and thus, the production cost is inevitably increased.
As another process for producing an opaque silica glass article, a process has been proposed wherein a highly purified crystalline silica powder is heated in an ammonia atmosphere and then the thus-ammoniated silica powder is heated and melted in an inert gas atmosphere to give an opaque silica glass article having an increased number of very fine bubbles, i.e., having a large total cross-sectional area of bubbles per unit volume of the opaque silica glass, and thus exhibiting an enhanced heat insulation (JP-A H7-61827 and JP-A H7-300341). However, this process has problems such that the density of opaque silica glass, and the diameter and amount of bubbles contained therein greatly varies depending upon the particle diameter and particle diameter distribution of raw material powder and the state of raw material powder charged in a vessel for fusion, and thus, the diameter and amount of bubbles in the surface portion and those in the central portion greatly differ from each other, and an opaque silica glass article having bubbles uniformly distributed therein is difficult to produce with good reproducibility.
As still another process for producing an opaque silica glass article, a process has been proposed wherein a finely divided powder of a foaming agent such as carbon or silicon nitride is incorporated in a siliceous raw material such as silica rock or stone, .alpha.-quartz or cristobalite, and the mixture is subjected to a flame fusion using an oxyhydrogen flame (JP-A H4-65328). The above-mentioned problems can be solved by this proposed process. However, the use of oxyhydrogen flame invites introduction of a hydroxyl group within silica glass which leads to reduction of the viscosity of molten glass and results in an opaque silica glass article not suitable as articles used for a long period of time at a high temperature, such as a jig for the production of semiconductor devices. Further, in this flame fusion step, the residence time of finely divided particles in the flame is very short, and the completion of reaction in the flame is difficult and it is possible that the foaming agent incorporated remains in the molten material as a foreign matter, and further that the siliceous raw material reacts with the forming gent with the result of undesirable coloration of the molten material.
It is said that, when a silica glass jig for the production of a semiconductor is cleaned after the use thereof, the bubbles exposed on the surface is removed, i.e., the surface is partly scraped down. To solve this problem, a procedure has been adopted for adhering a protective transparent silica glass film of a predetermined shape on the surface by heating with oxyhydrogen flame or in an electric furnace.
For the flange provided at the base of a furnace tube of a heating furnace for a silicon wafer, a heat insulating property as well as a sealing property are required to stably control the atmosphere within the furnace tube. Conventional opaque silica glass flanges have a rough surface due to the presence of bubbles and thus, even where a packing is used, a complete seal cannot be attained. For overcoming this defect, a flange having an opaque portion with good heat insulating property and a transparent portion and with good sealing property is suitable.
Several processes have been proposed for producing the flange having an opaque portion with good heat insulating property and a transparent portion with good sealing property is suitable. As examples of such processes, there can be mentioned (1) a process for fusion-bonding a transparent silica glass article to an opaque silica glass article, (2) a process wherein a powdery raw material for an opaque silica glass is added to a transparent silica glass article and the combination thereof is fusion-bonded, (3) a process wherein a powdery raw material for an opaque silica glass and a powdery raw material for a transparent silica glass are melted, and (4) a process wherein a surface portion of an opaque silica glass article containing bubbles therein is melted whereby bubbles within the surface portion is removed and thus the surface portion is rendered transparent.
The above-recited processes have the following problems. Namely, in the process of (1), at the step of fusion-bonding, bubbles are liable to occur at the interfacial boundary between the transparent silica glass portion and the opaque silica glass portion thereof. In general the adhesion between the transparent portion and the opaque portion thereof is not sufficient and the adhered transparent portion and opaque portion are liable to be separated. Further when the shape of the opaque silica glass article is complicated, the transparent silica glass becomes very difficult to fabricate and to fusion-bond to the opaque silica glass.
In the process of (2), bubbles do not readily occur at the interfacial boundary between the two silica glass portions, but the powdery raw material for the opaque silica glass portion shrinks in the course from the fusion bonding step to the completion of vitrification, and thus the resulting silica glass article is liable to warp. More specifically, JP-A H7-300326 discloses a process wherein a transparent silica glass article is placed in a heat-resistant mold, a powdery raw material for forming opaque silica glass is superposed upon the transparent silica glass article, and then the combined material is subjected to fusion bonding in an inert gas atmosphere to give a silica glass article having an opaque silica glass layer and a transparent silica glass layer. In this process, when the superposed powdery raw material containing an inert gas amoung the particles is melted and vitrified, the inert gas contained among the particles is entrapped within the molten material and becomes bubbles in the resulting glass article. However, the amount of gas derived from the raw material, the number and diameter of bubbles occasionally vary and the bubbles are difficult to uniformly distribute within the glass, and sometimes an inert gas introduced at the step of fusion bonding becomes part of the bubbles within the glass. Therefore, the bubbles within the opaque portion of the silica glass article are difficult to control.
In the process of (3), the gas contained in the powdery raw material for forming an opaque portion partly penetrates into the powdery raw material for forming a transparent portion with the result of occurrence of bubbles in the vicinity of the interfacial boundary. Further the opaque silica glass portion and the transparent silica glass portion, both of which shrink in the course from fusion bonding to the completion of vitrification, exhibit different shrinkages, and thus, the resulting silica glass article tends to warp.
In the process of (4), it is difficult to melt uniformly in thickness the surface portion of the bubble-containing opaque silica glass article, and further to deaerate the molten surface portion of a satisfying extend.